Transport in nanoporous media differs from ordinary transport in bulk media. Nanoporous material refers generally to material having one or more pores less than one micrometer in size. The differences arise largely because the interactions between the pore surface and the molecule being transported become increasingly important as the dimensions of the pore approach the size of the molecule.
Conventional nanopore models, such as dialysis membranes or polymeric woven-fiber membranes, contain a large array of pores with polydisperse structural parameters; that is, nanopores in these membranes exhibit a wide distribution in either shape, size, or surface chemistry. Quantitative data analysis for such models is difficult to implement because a complete description of a polydisperse structure is almost impossible without invoking many approximations and assumptions.
More recently, materials containing arrays of pores with one or more monodisperse structural parameters have been reported: for example, membranes derived from etched polycarbonate (Nucleopore) or porous alumina membranes and porous structures fabricated from monodisperse nanoscopic and mesoscopic objects. Membrane models containing monodisperse pore-arrays do have some drawbacks. First, it is difficult to ensure structural uniformity in arrays containing 10 or more pores (assuming a pore diameter of 100 nm and a sample area of 1 cm2); the problem is exacerbated as the pore dimensions become very small. Second, under steady-state conditions, only a time-averaged transport rate can be determined because individual single-pore transport events cannot be temporally resolved from each other. Thus, statistical distribution in transport rate cannot be retrieved using an array-pore membrane model.
Single-pore membranes represent a new type of structural model for studying mass-transport kinetics. Since the number of variables required for complete structural description is less than for array-pore membranes, single-pore membranes are more useful for directly testing specific predictions of theory. Single-pore membranes allow measurement of the temporal response of a single pore, which is useful for obtaining stochastic information about transport parameters or for investigating time-dependent properties such as voltage- or chemically induced gating. Single nanopores consisting of membrane proteins have been studied previously. However, these protein channels are dynamically complex structures and may not be good models for testing existing theories.
One challenge often encountered when relying on single-pore membrane models for transport studies is that very few methods exist that allow convenient fabrication of single-pore membranes with pore dimensions on the nanometer scale. Current methods for producing single-pore membranes generally fall into two categories: the first includes methods based on optical or e-beam lithography, and the second includes the methods based on radiation damage (e.g., Nucleopore membranes track-etched by low density, high-energy fission fragments or inorganic membranes such as a sapphire membrane drilled by a focused laser beam).